1. Field of the Invention
The present invention relates to evaporated fuel processing systems in which a canister is charged with evaporated fuel generated in the fuel tank via a charge passage and the evaporated fuel purged from the canister is supplied to the intake passage of an engine via a purge passage.
2. Description of the Prior Art
An evaporated fuel processing system is provided in order to prevent evaporated fuel generated in the fuel tank of an automobile from diffusing into the atmosphere. The evaporated fuel processing system comprises a canister containing active carbon, which can be charged with and purged of the evaporated fuel. The fuel tank and the canister are connected to each other via a charge passage, and the evaporated fuel generated in the fuel tank is supplied to the canister via the charge passage and adsorbed by the active carbon. The canister is also connected to the intake passage of the engine via a purge passage, the fuel which has been adsorbed by the active carbon is purged by air which is taken into the canister through an atmosphere connection hole by means of the negative intake pressure, and the purged evaporated fuel is supplied to the intake passage of the engine via the purge passage.
Japanese Patent Application Laid-open No. 6-185420 discloses such an evaporated fuel processing system in which, after a pathway from the fuel tank to the intake passage of the engine has been depressurized by the negative intake pressure, a charge control valve provided in the charge passage is closed to seal the fuel tank (and the charge passage upstream of the charge control valve) at a reduced pressure, and detection of a leakage fault is attempted by monitoring changes in the internal pressure of the fuel tank thereafter. When a leakage fault is detected as an increase in the internal pressure of the fuel tank, both the charge control valve and the purge control valve are opened and the evaporated fuel within the fuel tank is sucked into the intake passage of the engine by means of the negative intake pressure to thereby prevent the evaporated fuel from diffusing into the atmosphere from the point where the leakage fault has occurred.
However, in the prior art, when a leakage fault is detected and the evaporated fuel within the fuel tank is sucked into the intake passage of the engine by means of the negative intake pressure, the atmosphere release control valve for opening and closing the atmosphere communication hole of the canister which is positioned between the fuel tank and the intake passage of the engine, is maintained in an open state. Therefore, the negative intake pressure of the engine is consumed by the intake of air from the atmosphere release control valve of the canister, and the negative intake pressure of the engine cannot be transmitted effectively to the point where the leakage fault has occurred, which is upstream of the atmosphere release control valve of the canister. As a result it is difficult to completely prevent the evaporated fuel from leaking from the point where the leakage fault has occurred.
The present invention has been conducted in view of the above mentioned circumstances, and when a leakage fault occurs in the evaporated fuel processing system of a fuel tank, it is an object of the present invention to reliably prevent the evaporated fuel from leaking from the point where the leakage fault has occurred.
In order to achieve the above-mentioned objective, in accordance with the present invention, an evaporated fuel processing system is proposed which comprises a fuel tank for holding fuel, a canister which can be charged with and purged of evaporated fuel, a charge control valve for opening and closing a charge passage which connects the fuel tank to the canister, a purge control valve for opening and closing a purge passage which connects the canister to an intake passage of an engine, and an atmosphere release control valve for opening and closing an atmosphere communication hole of the canister. A control means is provided which detects a leakage fault in the fuel tank or the charge passage upstream of the charge control valve, and when a leakage fault is detected the control means opens the charge control valve and the purge control valve and closes the atmosphere release control valve.
In accordance with the above-mentioned system, when a leakage fault is detected the charge control valve and the purge control valve are opened to transmit the negative intake pressure of the intake passage of the engine to the point where the leakage fault has occurred, the evaporated fuel is sucked into the intake passage of the engine by means of the negative intake pressure, and thus leakage from the point where the leakage fault has occurred can be prevented. Since the atmosphere release control valve of the canister is maintained in a closed state during this period, the negative intake pressure of the engine cannot be consumed by the intake of air from the atmosphere release control valve, and the negative intake pressure of the engine can be transmitted efficiently to the point where the leakage fault has occurred to effectively prevent the evaporated fuel from leaking from the point where the leakage fault has occurred.
Furthermore, an evaporated fuel processing system according to the present invention comprises a pressure detecting means for detecting the internal pressure of the fuel tank or the charge passage upstream of the charge control valve, and when the amount of leakage detected by the control means is less than or equal to a predetermined value, the control means controls the degree of opening of the purge control valve on the basis of the pressure detected by the pressure detecting means such that the internal pressure of the fuel tank becomes slightly negative.
In accordance with the above-mentioned system, since in the case where the amount of leakage is not more than a predetermined value the degree of opening of the purge control valve is controlled on the basis of the pressure detected by the pressure detecting means so that the internal pressure of the fuel tank is slightly negative. Thus the amount of evaporated fuel sucked into the intake passage of the engine can be minimized while at the same time preventing the leakage of evaporated fuel from the point where the leakage has occurred, and the time required for the canister to become fully charged can thus be extended.
Furthermore, an evaporated fuel processing system according to the present invention comprises a pressure detecting means for detecting the internal pressure of the fuel tank or the charge passage upstream of the charge control valve, and when the control means detects an open failure of the charge control valve, the control means controls the degree of opening of the purge control valve on the basis of the pressure detected by the pressure detecting means such that the internal pressure of the fuel tank becomes slightly negative.
In accordance with the above-mentioned system, where there is an open failure in the charge control valve, the degree of opening of the purge control valve is controlled on the basis of the pressure detected by the pressure detecting means. Thus the internal pressure of the fuel tank becomes slightly negative, and the time required for the canister to become fully charged due to the additional supply of evaporated fuel from within the fuel tank to the canister can be extended.